Rotator driving device, image forming apparatus using the rotator driving device, and method of driving rotator

ABSTRACT

Rotational force of a driving motor is transferred to a photosensitive drum via a planetary-roller reduction device, so that the photosensitive drum is rotated. A speed detector set on the shaft of the photosensitive drum detects the rotational speed of the photosensitive drum. In accordance with the detected rotational speed of the photosensitive drum, the rotational speed of the driving motor is adjusted so that the photosensitive drum is rotated at a constant speed.

[0001] This application is based on applications No. 11-64051, No.11-89806, and No. 11-89807 filed in Japan, the contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] (1) Field of the Invention

[0003] The present invention relates to a rotator driving device forrotationally driving a rotator, such as a photosensitive drum providedin a copier, color printer, facsimile, or the like, and also relates toan image forming apparatus using the rotator driving device and a methodof rotating such a rotator.

[0004] (2) Related Art

[0005] In general, a high degree of uniformity is required to rotate arotator. The reason why the high-degree uniformity is required is givenfor a case of a photosensitive drum provided as a rotator in, forexample, a copier.

[0006] The photosensitive drum is rotated in one direction, and a laserbeam scans the surface of the photosensitive drum in the direction ofthe axis of rotation i.e. the main scanning direction) at every scanningcycle. As a result of scanning, an electrostatic latent image is formedon the surface of the photosensitive drum. If the rotational speed ofthe drum is unstable, that is, if the drum has nonuniformity inrotation, nonuniformity accordingly occurs in the distances between thescanning lines. This causes inconsistency in the print density on areproduced image and so deteriorates the image quality. For this reason,a high level of uniformity is required in the rotational speed of thephotosensitive drum.

[0007] The nonuniformity in rotation is caused by various factors. Itmay be caused due to eccentricity of the photosensitive drum and thusoccur in a cycle of one rotation of the drum. The nonuniform rotation ata low frequency may be caused by torsion of a motor shaft and a loadshaft. Meanwhile, the nonuniform rotation at a high frequency may becaused by improper engagement of gearwheels or a timing belt.

[0008] To raise the level of uniformity in rotation, a reduction devicehaving a planetary roller instead of gearwheels or a timing belt hasbeen used. By means of this reduction device, the nonuniformity inrotation at a high frequency can be eliminated. This technique isdisclosed in Japanese Laid-Open Patent Application Nos. 5-53381 and5-180290, for example.

[0009] The nonuniformity in rotation at a high frequency that is causedby the improper engagement of the gearwheels used as a reduction devicecan be improved by a rotator driving device that has a planetary roller.However, there is a possibility that the nonuniformity in rotation at alow frequency would increase due to factors, such as a skid of theplanetary roller. This problem occurs not only to a photosensitive drumprovided in an image forming apparatus. It commonly occurs to otherkinds of rotator driving devices that each have a reduction device witha planetary roller and that require uniformity in the rotational speed.

[0010] Additionally, a rotator driving device conventionally has to beprovided for each rotator, such as a photosensitive drum or developingroller. Therefore, these rotator driving devices to be equipped in anapparatus, such as a copier, occupy a large space, thereby making hardto manufacture the apparatus as compact as possible.

[0011] To realize a compact rotator driving device, Japanese Laid-OpenPatent Application No. 4-245261, for instance, discloses a technique ofrotationally driving a plurality of rotators using only one drivingsource. To be more specific about this technique, a servomotor that hasa reduction device with a planetary roller directly drives one of theprovided rotators, and then a rotational force of the servomotor issequentially transferred to the other rotators via idler rollers.

[0012] In this case, however, attention should be paid to that therotators are mechanically connected to each other via the idler rollers,meaning that the neighboring rotator and idler roller are in contactwith each other. With this construction, if low frequency elements, suchas nonuniformity in rotation or vibration, occur to one of the rotators,these elements may be transferred to the other rotators via the idlerrollers. In addition, if such low frequency elements occur to one of theidler rollers, the following rotators would be adversely affected bythis increased nonuniformity. Thus, the aim of realizing a drivingdevice that has a high degree of precision cannot be achieved.

SUMMARY OF THE INVENTION

[0013] The first object of the present invention is to provide a novelrotator driving device and an image forming apparatus I-using therotator driving device. The rotator driving device of the presentinvention eliminates nonuniformity in rotation at a low frequency thatoccurs to a rotator rotated by a reduction device with a planetaryroller, so that the rotational speed of the rotator can be more stable.A photosensitive drum provided as a rotator in the image formingapparatus is rotated at a constant speed using the rotator drivingdevice, so that high-quality images can be reproduced.

[0014] The second object of the present invention is to provide a novelrotator driving device, an image forming apparatus using the rotatordriving device, and a novel method of driving a rotator. The rotatordriving device of the present invention can respond to the trend towardsize reduction, and prevent nonuniformity in rotation or mechanicalvibration of a rotator from adversely affecting another rotator so thateach rotator can rotate at a constant rotational speed.

[0015] The first object of the present invention can be achieved by arotator driving device that rotationally drives a rotator, the rotatordriving device being made up of: a motor; a planetary-roller reductionunit that outputs a rotational speed that is reduced with respect to arotational speed of the motor, and transfers a rotational drive force ofthe motor to the rotator; a speed detector that detects a rotationalspeed of the rotator; and a controller that controls the rotationalspeed of the motor in accordance with a detection result obtained by thespeed detector.

[0016] With this construction, the rotational drive force of the motorserving as the rotational driving source is transferred to the rotatorvia the planetary-roller reduction unit. Also, the controller controlsthe rotational speed of the motor in accordance with the detectionresult obtained by the speed detector. Consequently, nonuniformity inrotation at high and low frequencies is eliminated, so that therotational speed of the rotator can be maintained constant.

[0017] The second object of the present invention can be achieved by arotator driving device that is provided in an image forming apparatusand that drives first and second rotators provided in the image formingapparatus, the rotator driving device being made up of: a driving unitthat includes a motor and supplies a rotational drive force of the motorto the first rotator; a speed detector that detects a rotational speedof the first rotator; a controller that controls a rotational speed ofthe motor in accordance with a detection result obtained by the speeddetector; and a drive branching unit that branches the rotational driveforce of the motor and transfers the branched rotational drive force tothe second rotator. It should be noted here that “branch” used in thepresent specification does not mean “separate” or “divide.” To be morespecific, even after a rotational drive force branches off using thedrive branching unit, the branched force is still the same as theoriginal rotational drive force in strength.

[0018] By means of this construction, the rotational drive force of themotor is transferred to the first rotator via the speed reducer whilethe rotational drive force branched by the drive branching unit istransferred to the second rotator. As such, the rotational drive forcecan be transferred to a plurality of rotators using only one rotationaldriving source. This leads to space saving, and the image formingapparatus can be manufactured compact. The rotational speed of the motoris controlled in accordance with the detected rotational speed of thefirst rotator. Thus, if nonuniformity in rotation occurs to the secondrotator, the nonuniformity is prevented from being transferred to thefirst rotator. As a result, the first rotator is always rotationallydriven at a constant speed.

[0019] The second object of the present invention can be also achievedby a rotator driving device that drives first and second rotators, therotator driving device being made up of: a motor that supplies arotational drive force to the first rotator; and a drive branching unitthat branches the rotational drive force, the branched rotational driveforce being used for driving the second rotator, wherein the drivebranching unit has a main rotating member and a slave rotating member,the main rotating member being set coaxial with the first rotator and sorotating as the first rotator rotates and the slave rotating memberbeing connected to the second rotator and rotating together with themain rotating member owing to a magnetic action exerted between the mainand slave rotating members.

[0020] With this construction, the rotational drive force can betransferred to a plurality of rotators using only one motor. This leadsto space saving and an image forming apparatus including the rotatordriving device can be manufactured compact. The drive branching unit iscomposed of main and slave rotating members, the slave rotating memberrotating together with the main rotating member owing to the magneticaction exerted between the main and slave rotating members. By means ofthis construction of the drive branching unit, nonuniformity in rotationoccurring to a rotator can be absorbed more as compared with a casewhere the rotational drive force branches off using a gear or the like.Consequently, the nonuniformity in rotation can be reliably preventedfrom adversely affecting another rotator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] These and other objects, advantages and features of the inventionwill become apparent from the following description thereof taken inconjunction with the accompanying drawings which illustrate a specificembodiment of the invention. In the drawings:

[0022]FIG. 1 shows an entire construction of a tandem-type digital colorcopier of a first embodiment;

[0023]FIG. 2 shows a construction for rotationally driving aphotosensitive drum in the first embodiment;

[0024]FIG. 3A is a cross-sectional view, taken along the plane of thedot-dash line A-A of FIG. 2, showing a construction of a reductiondevice with a planetary roller of the first embodiment;

[0025]FIG. 3B is a cross-sectional view, taken along the plane of thedot-dash line B-B of FIG. 3A, showing the construction of the reductiondevice with the planetary roller;

[0026]FIG. 4 is a block diagram showing a construction of a rotationcontrol unit of the first embodiment;

[0027]FIG. 5 is a flow chart of an operation performed by a CPU of therotation control unit for adjusting the rotational speed of a motor;

[0028]FIGS. 6A and 6B show examples of a setting position of arotational speed detector, aside from the position described in thefirst embodiment;

[0029]FIG. 7 shows a construction of a rotator driving device of asecond embodiment;

[0030]FIG. 8 is a graph showing a transfer characteristic of a reductiondevice provided in the reduction device with a planetary roller shown inFIG. 7;

[0031]FIG. 9 is a flow chart of an operation performed for adjusting therotational speed of a motor provided in the rotator driving device shownin FIG. 7;

[0032]FIG. 10 shows a construction of a rotator driving device of athird embodiment;

[0033]FIG. 11 is a cross-sectional view, taken along the plane of theline C-C of FIG. 10, showing a drive branching unit provided in therotator driving device; and

[0034]FIG. 12 is a perspective view showing a construction of thedrive-dividing unit shown in FIG. 10.

DESCRIPTION OF PREFERRED EMBODIMENT

[0035] The following is a description of embodiments of an image formingapparatus that includes a rotator driving device of the presentinvention, with reference to the drawings. In the embodiments, atandem-type digital color copier (simply referred to as the “copier”hereinafter) is used as an example of such image forming apparatus.

First Embodiment

[0036] 1. Entire Construction of the Copier

[0037]FIG. 1 is a cross-sectional view showing the entire constructionof a copier 1 of the present embodiment. The copier 1 includes imageforming units 30C, 30M, 30Y, and 30K that are set above a transportingbelt 14 along its length. The transporting belt 14 is horizontally setin a lower space of an enclosure 10. Below the transporting belt 14, apaper feeding cassette 11 is set at the lowermost position of theenclosure 10 and can be freely slid in and out of the copier 1. Arecording sheet S is taken by a pick-up roller 21 from the paper feedingcassette 11 and then carried to the transporting belt 14 by means oftransporting rollers 22 to 25. The transporting belt 14 transports therecording sheet S, and the image forming units 30C to 30K sequentiallytransfer toner images for reproduction colors, i.e. cyan, magenta,yellow, and black, onto the recording sheet S. The toner images aresuperimposed on the recording sheet S to form a full-color image. Thereproduction colors are respectively referred to as C, M, Y, and Khereinafter and components related to these colors are assigned numeralswith a corresponding C, M, Y, or K.

[0038] The copier 1 further includes an image reading unit 15 at theupper part of the enclosure 10. The image reading unit 15 opticallyreads image data of an original document using a scanner and linearfull-color sensor (CCD sensor) so as to perform photoelectricconversion. Specifically, the image reading unit 15 converts the readimage data into electric signals for each of primary colors red (R),green (G), and blue (B). After the conversion, the image reading unit 15transmits the electric signals as the image data to an image processingunit 16.

[0039] The image processing unit 16 performs corrective processes on theimage data received from the image reading unit 15 for each of R, G, andB. Following this, the image processing unit 16 separates the image datainto gradation data for each reproduction color, C, M, Y, and K, andtemporarily stores the gradation data into a memory. Then, the imageprocessing unit 16 reads the data for each reproduction color andconverts the data into a driving signal for driving a correspondinglaser diode. The driving signals are transferred to optical units 40C to40K respectively set above the image forming units 30C to 30K.

[0040] The optical units 40C to 40K respectively drive the laser diodesin accordance with the driving signals (that is, the image data)transferred from the image processing unit 16. As a result, each laserdiode performs light modulation and emits a light-modulated laser beam.By scanning the laser beams across the corresponding surfaces ofphotosensitive drums 31C to 31K in the main scanning direction,electrostatic latest images are formed on the surfaces of thephotosensitive drums 31C to 31K. The optical units 40C to 40K have thesame construction and, therefore, an explanation is given only for theoptical unit 40C, as one example.

[0041] The optical unit 40C is composed of a laser diode 41C, a polygonmirror 42C, an f-θ lens (not illustrated), and redirecting mirrors (notillustrated). The laser diode 41C is driven to perform light modulationin accordance with the driving signal outputted from the imageprocessing unit 16 and emits a light-modulated laser beam. The emittedlaser beam is reflected off the polygon mirror 42C that is rotationallydriven by a motor (not illustrated), and passes through the f-θ lens.After this, the laser beam is sequentially reflected off the redirectingmirrors and then scans the surface of the photosensitive drum 31C in themain scanning direction.

[0042] The image forming units 30C to 30K respectively have thephotosensitive drums 31C to 31K as main components that are rotated inthe direction of the arrows A. Around the photosensitive drums 31C to31K, developing units 38C to 38K and sensitizing chargers 39C to 39K arerespectively provided. Before the exposure by the laser beam, each ofthe sensitizing chargers 39C to 39K uniformly charges the correspondingsurface of the photosensitive drums 31C to 31K. With this charged stateof the photosensitive drums 31C to 31K, the laser beams respectivelyscan the surfaces of the photosensitive drums 31C to 31K, so thatelectrostatic latent images are formed on the surfaces of the drums 31Cto 31K. The developing units 38C to 38K respectively develop theelectrostatic latent images into visible toner images. Accordingly, eachof the image forming unit 30C to 30K has a unit construction includingthe stated it components to achieve image formation according to aso-called “electrostatic copying method.”

[0043] The developing units 38C to 38K are respectively provided withdeveloping rollers 37C to 37K as rotators. The developing rollers 37C to37K respectively supply the photosensitive drums 31C to 31K with tonersC, M, Y, and K provided in the developing units 38C to 38K as developerscorresponding to the light-modulated colors of the optical units 40C to40K.

[0044] Transfer chargers 17C to 17K are set underneath the transportingbelt 14 at transfer positions located directly under the photosensitivedrums 31C to 31K.

[0045] The transporting belt 14 runs over a driving roller 18, a slaveroller 19, and a tension roller 20. The driving roller 18 isrotationally driven by a motor (not illustrated) in the direction of thearrow B. Together with the rotation of the driving roller 18, thetransporting belt 14 moves in the direction of the arrow C. Here, therotational speed of the motor is controlled so that the moving speed ofthe transporting belt 14 is equal to the circumferential speed of eachphotosensitive drum 31C to 31K when image formation is performed. Thetension roller 20 is energized in the direction of the arrow D by atensile spring (not illustrated) and keeps the tension of thetransporting belt 14 constant.

[0046] With the application of electric fields of the transfer chargers17C to 17K, the toner images formed on the surfaces of thephotosensitive drums 31C to 31K are transferred onto the recording sheetS transported by the transporting belt 14.

[0047] After the toner image transfer, the recording sheet S istransported by the transporting belt 14 to a fixing unit 26 which fixesthe transferred toner image onto the recording sheet S. Finally, therecording sheet S is discharged onto a discharging tray 13.

[0048] Registration sensors 32C to 32K are respectively set before thephotosensitive drums 31C to 31K in the transport direction of therecording sheet S as shown in FIG. 1, and detect the leading edge of therecording sheet S which is transported by the transporting belt 14. Inaccordance with the detection timing, exposure by the laser beam for thecorresponding photosensitive drum 31C to 31K is started. Eachregistration sensor 32C to 32K is set so that a distance between thedetection position of the registration sensor and the transfer positionof the corresponding photosensitive drum is longer than acircumferential distance of the photosensitive drum between the emittingposition of the laser beam and the transfer position measured in therotational direction of the photosensitive drum.

[0049] The photosensitive drums 31C to 31K are respectively rotationallydriven by driving motor units 33C to 33K (see FIGS. 2 and 4). Eachdriving motor unit 33C to 33K has a reduction device with a planetaryroller. Hereinafter, a reduction device with a planetary roller isreferred to as the “planetary-roller reduction device.”

[0050] 2. Construction for Rotating the Photosensitive Drums

[0051] The copier 1 that reproduces color images using the statedcomponents also has a construction to control the rotational speeds ofthe photosensitive drums 31C to 31K. With this construction,electrostatic latent images are formed on the surfaces of the drums 31Cto 31K with high fidelity. The photosensitive drums 31C to 31K have thesame construction and, therefore, the detailed explanation of theconstruction is given only for the photosensitive drum 31C as oneexample.

[0052]FIG. 2 shows the photosensitive drum 31C and the construction forrotationally driving the drum 31C, and also shows the optical unit 40C.The photosensitive drum 31C is rotationally driven by the driving motorunit 33C. The driving motor unit 33C includes a driving motor 34C and aplanetary-roller reduction device 35C.

[0053] The driving motor 34C is a stepping motor, and the rotationalspeed of the driving motor 34C is variably controlled in accordance witha driving pulse inputted into the motor 34C from a rotation control unit50 (see FIG. 4). The rotation control unit 50 controls each rotation ofthe photosensitive drums 31C to 31K and will be described in detaillater in this specification. The rotational force of the driving motor34C is transferred to the planetary-roller reduction device 35C by amotor shaft 341C.

[0054] The planetary-roller reduction device 35C is a well-knownreduction device, and is basically composed of a sun roller, planetaryrollers, and a carrier. The planetary rollers are in contact with theouter surface of the sun roller and rotate, and the carrier supports theplanetary rollers so that the planetary rollers can smoothly rotate. Asthe sun roller rotates, the planetary rollers transfer the rotationalforce of the sun roller to the carrier by friction drive. Here, adesired speed increasing/reducing ratio can be obtained by appropriatelyarranging the sun roller, the planetary rollers, and the carrier and byadjusting each perimeter of the rollers. As understood from theconstruction of the reduction device 35C, the rotational force istransferred since the outer surfaces of the rollers are in contact witheach other. Thus, the force transfer can be smoothly performed, so thatvibration at a high frequency will not occur even when a gearwheel, suchas a reduction gear, is used. However, skids may occur to the surfaceson the rollers that are in contact with each other, meaning that thenonuniformity in rotation at a low frequency will increase.

[0055] The detailed construction of the planetary-roller reductiondevice 35C of the present embodiment is explained with reference toFIGS. 3A and 3B. FIG. 3A is a cross-sectional view taken along the planeof the dot-dash line A-A of FIG. 2. FIG. 3B is a cross-sectional view,taken along the plane of the dot-dash line B-B of FIG. 3A, that isviewed in the direction of the arrows.

[0056] As shown in FIG. 3A, planetary rollers 352C, 353C, and 354C areprovided around the outer surface of the motor shaft 341C that serves asa sun roller. As shown in FIG. 3B, the planetary rollers 352C and 354Care axially supported by axial members 3521C and 3541C that stand on aside of the carrier 355C. Although not illustrated in this figure, theplanetary roller 353C is also axially supported by an axial member3531C. The outer surfaces of the planetary rollers 352C to 354C are incontact with the inner surface of a housing 357C. As the driving motor34C rotates, each of the planetary rollers 352C to 354C revolves aroundthe motor shaft 341C along the inner surface of the housing 357C,rotating on its axis. Together with these rotations of the rollers 352Cto 354C, the carrier 355C that axially supports the rollers 352C to 354Crotates at a speed lower than a speed at which the motor shaft 341Crotates.

[0057] Each of the planetary rollers 352C to 354C is formed by coveringthe outer surface of its metal shaft with elastic material to avoidskids. However, it is impossible to completely prevent skids.

[0058] As shown in FIG. 3B, an output shaft 356C for transferring therotational force of the carrier 355C is provided on the other side ofthe carrier 355C, opposite to the side on which the axial members 3521Cto 3541C stand. The motor shaft 341C, the planetary rollers 352C to354C, and the carrier 355C are set in the housings 357C and 358C thatare secured to each other by screws 3581C, 3582C, and 3583C. The housing357C is secured to an inner wall of the copier 1 by a supporting member(not illustrated).

[0059] In FIG. 2, the output shaft 356C of the carrier 355C is coupledto a rotational shaft 311C of the photosensitive drum 31C via a coupling27C so that the output shaft 356C can be easily detached/reattached.With this construction, the driving motor 34C rotationally drives thephotosensitive drum 31C eventually. By the provision of the coupling 27,the photosensitive drum 31C is removable from the copier 1. Thus, easymaintenance can be achieved by removing the photosensitive drum 31C fromthe copier 1.

[0060] A rotational speed detector 28C is a pulse encoder that is set onthe shaft 311C at a position opposite to the driving motor unit 33C,with the photosensitive drum 31C in between them. The rotational speeddetector 28C detects the rotational speed of the shaft 311C, and outputsa pulse at a frequency corresponding to the detected rotational speed tothe rotation control unit 50. Note that the rotational speed of theshaft 311C refers to the rotational speed of the photosensitive drum31C. Hereinafter, the pulse outputted from the detector 28C is referredto as the “speed detection signal.” As the rotational speed detector28C, various other types of components, such as a tachometer generator,can be used.

[0061] 3. Construction of the Rotation Control Unit 50

[0062]FIG. 4 is a block diagram showing the construction of the rotationcontrol unit 50.

[0063] As shown in FIG. 4, the rotation control unit 50 is composed of aCPU 51, pulse generating units 52C to 52K, driver units 53C to 53K, aRAM 58, and a ROM 59. Aside from the units provided in the rotationcontrol unit 50, a main control unit 61, rotational speed detectors 28Cto 28K, and the registration sensors 32C to 32K are also connected tothe CPU 51. The main control unit 61 comprehensively controls anoperation performed by the entire copier 1. The rotational speeddetectors 28C to 28K respectively detect the rotational speeds of thephotosensitive drums 31C to 31K. The main control unit 61 is furtherconnected to a control unit 400 that controls the optical units 40C to40K, the image reading unit 15, and the image processing unit 16.

[0064] The components included in the rotation control unit 50 aredescribed as follows. Note that components that are respectivelyprovided for the reproduction colors C, M, Y, and K have the samefunction, and therefore the description is given only for the componentsassociated with cyan as one example.

[0065] The pulse generating unit 52C generates a motor driving pulsethat has a cycle corresponding to a frequency outputted from the CPU 51or a frequency outputted by a crystal oscillator provided in therotation control unit 50. Then, the pulse generating unit 52C outputsthe motor driving pulse to the driver unit 53C.

[0066] The driver unit 53C rotationally drives the driving motor 34C ata rotational speed that corresponds to the received motor driving pulse.This means that the rotational speed of the driving motor 34C variesaccording to on the cycle of the motor driving pulse generated by thepulse generating unit 52C. To be more specific, the rotational speed ofthe driving motor 34C increases when the cycle of the motor drivingpulse is short, while it decreases when the cycle is long. In accordancewith an instruction from the CPU 51, the driver unit 53C starts or stopspassing a current through the driving motor 34C.

[0067] The CPU 51 adjusts the rotational speed of the driving motor 34Cin accordance with programs stored in the ROM 59, so that thephotosensitive drum 31C is rotated at a desired rotational speed.

[0068] The adjusting operation performed by the CPU 51 is explained,with reference to the flow chart shown in FIG. 5. FIG. 5 is a flow chartshowing the operation performed by the CPU 51 for adjusting therotational speed of the driving motor 34C.

[0069] On receiving an instruction that is issued from the control unit61 when a user instructs to start a copy operation, the CPU 51 isactivated (step S501). The CPU 51 reads an initial value of frequencyfrom the ROM 59 and outputs it to the pulse generating unit 52C. Thepulse generating unit 52C generates a motor driving pulse correspondingto the initial value of frequency to rotate the driving motor 34C (stepS502). After the driving motor 34C starts rotating, the rotational speeddetector 28C sends a speed detection signal to the CPU 15. The speeddetection signal indicates the rotational speed of the rotational shaft311C, i.e. the rotational speed of the photosensitive drum 31C.

[0070] The CPU 51 refers to an internal timer and judges whether apredetermined period of time has been elapsed (step S503). If it has(“YES” in step S503), the CPU 51 detects a frequency of the speeddetection signal and judges whether the frequency is within anappropriate range that is stored in the ROM 59 beforehand (step S504).This appropriate range includes a predetermined level of tolerance withrespect to a frequency of a speed detection signal that is expected tobe outputted by the rotational speed detector 28C when thephotosensitive drum 31C rotates at a desired speed. This predeterminedlevel of tolerance is properly determined so that the human eye cannotperceive inconsistency in the print density or color displacements on areproduced image that may occur due to the nonuniformity in rotation. Ifthe frequency value of the speed detection signal exceeds the maximumvalue of the appropriate range, the CPU 51 judges that the currentrotational speed of the photosensitive drum 31C is higher than thedesired speed. Meanwhile, if the frequency value of the detection signalis below the minimum value of the appropriate range, the CPU 51 judgesthat the current rotational speed of the photosensitive drum 31C islower than the desired speed.

[0071] If judging the rotational speed of the drum 31C is higher thanthe desired speed (“>” in step S505), the CPU 51 instructs the driverunit 53C to stop passing the current through the driving motor 34C sothat the rotational speed of the driving motor 34C will decrease (stepS507) . On the contrary, if judging the rotational speed of the drum 31is lower than the desired speed (“<” in step S505), the CPU 51 instructsthe driver unit 53C to start passing the current through the drivingmotor 34C so that the rotational speed of the driving motor 34C willincrease (step S506).

[0072] The CPU 51 repeats the stated processes (steps S503 to S507)until receiving an instruction from the main control unit 61 to stoprotating the photosensitive drum 31C. On receiving the instruction(“YES” in step S508), the CPU 51 terminates this processing.

[0073] As clearly understood from the above explanation, for the copier1 of the present embodiment, the nonuniformity in rotation at a highfrequency can be reduced using the planetary-roller reduction device. Atthe same time, the nonuniformity in rotation at a low frequency thatincreases because of the provision of the planetary-roller reductiondevice can be reduced through the feedback control on the rotationalspeed of the driving motor using the rotational speed detector. Thefeedback control is more suitable for the solution of the nonuniformityin rotation at a low frequency than the nonuniformity in rotation at ahigh frequency. By taking two different measures against thenonuniformity in rotation at high and low frequencies, these twodifferent types of nonuniformity in rotation can be effectively reduced.Consequently, each photosensitive drum 31C to 31K rotates at a constantspeed and nonuniformity will not occur in the distances between thescanning lines in the sub-scanning direction. As a result, a color imagewithout color displacements can be reproduced.

[0074] In the present embodiment, the rotational speed detector 28C isset on the shaft 311C at the position opposite to the driving motor unit33C, with the photosensitive drum 31C in between them. Each settingposition of the rotational speed detectors 28C to 28K is not limited tothis position.

[0075]FIGS. 6A and 6B each show an example of a setting position of therotational speed detectors 28C to 28K, aside from the position shown inFIG. 2. Note that both FIGS. 6A and 6B show the examples only for thecomponents associated with cyan as one example.

[0076] In FIG. 6A, a rotational speed detector 88C is set on the shaft311C of the photosensitive drum 31C and located between the coupling 27Cand the body of the photosensitive drum 31C. In FIG. 6B, a rotationalspeed detector 89C serves as a part of the driving motor unit 33C and isset on the output shaft 356C of the planetary-roller reduction device35C. The detector 89C is located at the upstream side of the coupling27C in the transfer direction in which the rotational force of thedriving motor 34C is transferred. In the case shown in FIG. 6B, afterthe photosensitive drum 31 is removed from the copier 1, the rotationalspeed detector 89C will remain on the output shaft (356C) side.

[0077] However, in the case shown in FIG. 6B, nonuniformity in rotationcaused by torsion of the output shaft 356C and the shaft 311C at thecoupling 27C cannot be detected. For this reason, if the rotationalspeed detector 89C is set at the position as shown in FIG. 6B, suchnonuniformity in rotation due to the torsion needs to be preventedthrough appropriate measures, such as enlarging each diameter of theoutput shaft 356C and the shaft 311C.

[0078] In the present embodiment, a frequency of a speed detectionsignal is referred so as to detect the rotational speed of the rotator(i.e. the photosensitive drum). Here, the control on the rotationalspeed of the rotator can be achieved with a higher degree of precisionby referring to both a frequency and a phase of the speed detectionsignal.

[0079] In the present embodiment, an explanation has been given for acase where a stepping motor is used as a driving source of the rotator.However, the present invention can be applied to cases where other kindsof motor, such as a DC motor, is used.

[0080] In the present embodiment, the rotational speed of the drivingmotor 34C is adjusted to a desired speed by starting and stopping thepassage of current through the driving motor 34C. However, therotational speed of the driving motor 34C may be adjusted by changing acycle of the motor driving pulse.

Second Embodiment

[0081] In the first embodiment, the photosensitive drums 31C to 31K arerotationally driven using the planetary-roller reduction devices. In thesecond embodiment, a photosensitive drum and a developing roller arerotationally driven using one driving motor. Note that a copier of thepresent embodiment is the same as the copier 1 of the first embodiment,except for a rotator driving device used for otationally driving thephotosensitive drum and the developing roller. Therefore, theexplanation for the same components of the copiers is omitted in thepresent embodiment. These same components are assigned the same numeralsas in the first embodiment.

[0082]FIG. 7 shows a construction of a driving device 510 forrotationally driving the rotators, i.e. the photosensitive drum and thedeveloping roller of the developing unit. This figure alsodiagrammatically shows a circuit construction for rotationally driving adriving motor 501. The driving device 510 is provided for each pair of aphotosensitive drum and a developing unit, meaning that four drivingdevices 510 are provided in the copier 1 of the present embodiment. Assuch, in FIG. 7, the components are assigned numerals without C, M, Y,or K.

[0083] A driving motor unit 500 is composed of a driving motor 501 thatis a rotator-driving source and a planetary-roller reduction device 502.A control board 505, on which circuit components are containedbeforehand, is fixed between the driving motor 501 and theplanetary-roller reduction device 502.

[0084] As the driving motor 501, a stepping motor, AC motor, DC motor,or servomotor can be used. In the present embodiment, a DC motor isused.

[0085] An output shaft 503 of the driving motor 501 serves as a sunroller. Three planetary rollers 504 (only two rollers 504 are shown inFIG. 7) are in contact with the outer surface of the output shaft 503.Each planetary roller 504 revolves around the output shaft 503, rotatingon its axis. An output shaft 71 supports the planetary rollers 504 atits bottom end in such a manner that the rollers 504 can freely rotate.The top end of the output shaft 71 serves as a final output shaft thattransfers force from the driving motor unit 500.

[0086] The reduction device of the present embodiment is not limited tothe planetary-roller reduction device 502. For example, a reduction gearmechanism or a belt transmission mechanism may be employed for thereduction device.

[0087] The top end of the output shaft 71 is connected to thephotosensitive drum 31 via a coupling 70, so that the rotational forceof the driving motor 501 is transferred to the photosensitive drum 31.The coupling 70, the planetary-roller reduction device 502, and theoutput shaft 71 comprise a first drive transfer unit 72.

[0088] The output shaft 71 is provided with a speed detector 73 fordetecting the rotational speed of the output shaft 71. The speeddetector 73 corresponds to the rotational speed detector 28C (or, 28M to28K) described in the first embodiment.

[0089] As the speed detector 73, various types of components, such 27 asa pulse encoder or tachometer generator, can be used. In the presentembodiment, a pulse encoder is used. Also, a position at which the speeddetector 73 detects the rotational speed of the output shaft 71 may befreely set.

[0090] Since a pulse encoder is used as the speed detector 73 in thepresent embodiment, the speed detection signal would be a detectionpulse signal fn shown as a rectangular wave in FIG. 2. The detectionpulse signal fn is inputted into a phase comparing unit 554, into whicha standard signal fr is also inputted from a standard signal generatingunit 552. The frequency of the standard signal fr is the same as afrequency of a detection pulse signal that is expected to be outputtedby the speed detector 73 when the photosensitive drum 31 rotates at apredetermined speed. According to an instruction given by a CPU 551, thestandard signal generating unit 552 outputs the standard signal fr.

[0091] The phase comparing unit 554 converts a phase difference betweenthe standard signal fr and the detection pulse signal fn into a voltagevalue Vb. Meanwhile, the detection pulse signal fn is converted into avoltage value Va by a frequency-voltage (F-V) converting unit 557, intowhich a standard voltage value Vr related to the standard signal fr isinputted.

[0092] A mixing unit 581 includes an integrator circuit using anoperational amplifier. The output voltage values Va and Vb are inputtedto one input terminal of the operational amplifier provided in themixing unit 581. The standard voltage value Vr is inputted to the otherinput terminal of the operational amplifier. An output voltage from themixing unit 581 varies in accordance with fluctuations in a differenceof the frequency and phase of the detection pulse signal fn with respectto the standard signal fr. More specifically, the output voltage fromthe mixing unit 581 depends on a difference between the value Vr and avalue calculated by Va+Vb (this addition value is referred to as “Vc”hereinafter). It should be noted here that the value Vc becomes equal tothe voltage value Vr when the photosensitive drum 31 rotates at thepredetermined speed. When the current rotational speed of thephotosensitive drum 31 is lower than the predetermined speed, a relationof the values Vc and Vr is expressed as Vc<Vr. When the currentrotational speed of the drum 31 is higher than the predetermined speed,the relation is expressed as Vr<Vc.

[0093] A switching control unit 582 sets an ON/OFF duty factor that isreferred to for turning ON or OFF a switching element 583 in accordancewith the output voltage from the mixing unit 581. Hereinafter, a factorrepresenting a period of time during which the switching element 583 isturned ON is referred to as the “ON factor.” Based on the ON/OFF dutyfactor, the switching control unit 582 controls the switching element583. The switching element 583 is inserted into a power supplying line584 that is connected to the driving motor 501. Note that the drivingmotor 501 is in turn connected to a power source (not illustrated). Whenthe switching element 583 is turned ON, a current passes through thepower supplying line 584, so that the driving motor 501 is rotationallydriven. Hence, when the ON factor that has been set for the switchingelement 583 by the switching control unit 582 is relatively great, therotational speed of the driving motor 501 is high.

[0094] Suppose that the current relation is detected as Vc<Vr based onthe output voltage from the mixing unit 581. In this case, the switchingcontrol unit 582 sets the ON factor of the switching element 583 greaterthan the current factor in order to increase the rotational speed of thephotosensitive drum 31. Suppose, on the other hand, that the currentrelation is detected as Vr<Vc. In this case, the switching control unit582 sets the ON factor smaller than the current factor in order todecrease the rotational speed of the photosensitive drum 31.

[0095] In the present embodiment, a DC motor is used as the drivingmotor 501. If a stepping motor is used as in the case of the firstembodiment, the driver unit 53C (or, 53M to 53K) shown in FIG. 4 can beused as a driving circuit for driving the stepping motor used as thedriving motor 501.

[0096] In addition to the output shaft 503, the driving motor 501 isfurther provided with a rotational shaft 506 that rotates together withthe output shaft 503 at out the same axis and extends opposite indirection to the output shaft 503, as shown in FIG. 7. The rotationalshaft 506 is connected to a second drive transfer unit 75 via a drivebranching unit 74. The rotational force of the rotational shaft 506 istransferred as a branched rotational force to the developing roller 37via the drive branching unit 74 and the second drive transfer unit 75.It should be noted here that “branch” used in the present specificationdoes not mean “separate” or “divide.” To be more specific, even after arotational force branches off at the drive branching unit, the branchedforce (that is to be transferred to the developing roller 37 in thepresent embodiment) is still the same as the original rotational forcein strength.

[0097] The drive branching unit 74 is composed of a drive gear 741 and areduction gear 742 to form a reduction gear mechanism. The drive gear741 is fixed to the rotational shaft 506. The reduction gear 742 mesheswith the drive gear 741 and is fixed to one end of a first transmissionshaft 93. As shown in FIG. 7, the first and second transmission shafts93 and 94 are supported by a pair of shaft bearing members 91 and 92 insuch a manner that the transmission shafts 93 and 94 freely rotate.

[0098] It should be note here that the construction of the drivebranching unit 74 is not limited to the reduction gear mechanism, andthat other kinds of mechanisms can be employed. For example, a belt orchain transmission mechanism can be employed.

[0099] The second drive transfer unit 75 is composed of first to thirdintermediate gears 751 to 753 and a slave gear 754. The firstintermediate gear 751 is fixed to the first transmission shaft 93, andthe second intermediate gear 752 is fixed to one end of the secondtransmission shift 94 and meshes with the first intermediate gear 751.The third intermediate gear 753 is fixed to the other end of the secondtransmission shaft 94, and the slave gear 754 is fixed to a shaft 47 ofthe developing roller 37 and meshes with the third intermediate gear753.

[0100] As is the case with the drive branching unit 74, the constructionof the second drive transfer unit 75 is not limited to the reductiongear mechanism, and other kinds of mechanism, such as a belt or chaintransmission mechanism, can be employed.

[0101] With the stated construction of the driving system, therotational force of the output shaft 503 of the driving motor 501 isdecelerated by the planetary-roller reduction device 502 and transferredto the photosensitive drum 31 at a high torque via the output shaft 71and the coupling 70.

[0102] Meanwhile, the branched rotational force from the rotationalshaft 506 is transferred to the shaft 47 of the developing roller 37 ata high torque via the second drive transfer unit 75. As a result, thedeveloping roller 37 is rotationally driven.

[0103] Accordingly, the photosensitive drum 31 and the developing roller37 do not have to be driven by separate motors. Both the drum 31 and theroller 37 can be driven by one motor, the driving motor 501, that isserving as the rotator driving source. Consequently, space for settingthe driving motor 501 can be saved as compared with a case where thephotosensitive drum 31 and the developing roller 37 are driven byseparate motors.

[0104] The rotational speed of the output shaft 71, i.e. the rotationalspeed of the photosensitive drum 31, is detected by the speed detector73. In accordance with the detected rotational speed, the feedbackcontrol is performed on the driving motor 501, so that even whennonuniformity in rotation occurs to the developing roller 37, thenonuniformity is prevented from being directly transferred to thephotosensitive drum 31. Therefore, the photosensitive drum 31 isrotationally driven at a constant speed.

[0105] Particularly in the present embodiment, the rotational force ofthe photosensitive drum 31 is not directly transferred to the developingroller 37 and vice verse, since the planetary-roller reduction device502 is set in a force transfer path between the photosensitive drum 31and the developing roller 37. Suppose that load fluctuations, such asvibration, occur to the developing unit 38 or that noise as loadfluctuations occurs to the driving branching unit 74 due to improperengagement of the gears 741 and 742. In such a case, the loadfluctuations are attenuated owing to the transfer characteristic of theplanetary-roller reduction device 502, so that the photosensitive drum31 can be effectively prevented from being adversely affected by theload fluctuations.

[0106]FIG. 8 is a graph showing the transfer characteristic of theplanetary-roller reduction device 502. This transfer characteristic canbe obtained beforehand by calculating a gain G of output noise from theplanetary-roller reduction device 502, the output noise resulting frominput noise from the driving motor 501 serving as the rotator drivingsource. The gain G is calculated by dividing the output noise by theinput noise. In accordance with this transfer characteristic of theplanetary-roller reduction device 502, a frequency of the loadfluctuations applied to the planetary-roller reduction device 502 is setso that a value of the gain G remains below 1.

[0107] When a frequency f1 of the load fluctuations occurring to thedeveloping unit 38 is set within a frequency range in which the gain Gis equal to 1, the output noise from the planetary-roller reductiondevice 502 is almost the same as the input noise from the driving motor501. Meanwhile, when the frequency f1 is set within a frequency range inwhich the gain G is greater than 1, the output noise from the reductiondevice 502 is greater than the input noise from the driving motor 501.In these two cases, the load fluctuations occurring to the developingunit 38 are transferred to the photosensitive drum 31 via theplanetary-roller reduction device 502 and so adversely affect therotation of the photosensitive drum 31. This may lead to nonuniformityin rotation and interfere with forming an excellent image.

[0108] Meanwhile, suppose that the frequency f1 is set within afrequency range in which the gain G is smaller than 1, such as G=0.4,that is, the frequency f1 is set higher than a frequency f shown in FIG.8. In this case, the transfer of the load fluctuations occurring to thedeveloping unit 38 can be attenuated. Thus, the rotation of thephotosensitive drum 31 is less prone to the load fluctuations, so that areproduced image with high quality can be ensured.

[0109] When the frequency f1 is fixed, a resonance frequency f0 may bevariably controlled using a flywheel or the like. Then, the frequency f1is set within a frequency range where the gain G is smaller than 1.

[0110] In the present embodiment, the rotational force of the rotationalshaft 506 of the driving motor 501 is transferred to the developingroller 37. For this transfer of force, space provided on the rotationalshaft (506) side of the driving motor 501 is used for setting the drivebranching unit 74. In this way, design flexibility can be increased inthe case of the construction explained in the present embodiment.

[0111] It should be obvious that branching of the rotational force fromthe driving motor 501 to the developing roller 37 can be performedbetween the planetary-roller reduction device 502 and the driving motor501. The force branch at this position can also achieve the statedattenuation effect on the load fluctuations by means of theplanetary-roller reduction device 502. For the attenuation effect, thedrive branching unit 74 may be set on the upstream side of theplanetary-roller reduction device 502 in the direction in which therotational force of the driving motor 501 is transferred.

[0112]FIG. 9 is a flow chart of the feedback control performed by eachdriving motor unit 500 (that is, 500C to 500K).

[0113] When the copier 1 is turned on (“Y” in step S101), the CPU 551instructs the standard signal generating unit 552 to generate thestandard signal fr and has the switching control unit 582 perform theswitching control on the driving motor 501 in accordance with the setON/OFF duty factor so as to drive the driving motor 501 (step S102). Asstated earlier, the set ON/OFF duty factor has been set beforehand forthe switching control to be first performed when the driving motor 501is driven.

[0114] Then, the detection pulse signal fn outputted from the speeddetector 73 is detected (step S103). This detection pulse signal fn isconverted into a voltage value Va by the F-V converting unit 557 (stepS104). Following this, the phase comparing unit 554 compares the phaseof the standard signal fr and that of the detection pulse signal fn, anda phase difference is found as an error pulse (step S105). The phasecomparing unit 554 then converts this error pulse into a voltage valueVb (step S106).

[0115] After this, the switching control unit 582 performs the switchingcontrol in the following steps S107 to S109, depending on the currentrelation between the value Vc (=Va+Vb) and the standard voltage valueVr.

[0116] As stated above, when the current relation is expressed as Vc<Vrin step S107, the switching control unit 582 increases the ON factor ofthe switching element 583 to accelerate the driving motor 501 so thatthe rotational speed of the photosensitive drum 31 increases (stepS108). Then, the processing proceeds to step S110. Meanwhile, when thecurrent relation is expressed as Vr<Vc in step S107, the switchingcontrol unit 582 decreases the ON factor of the switching element 583 todecelerate the driving motor 501 so that the rotational a speed of thedrum 31 decreases (step S109). Then the processing proceeds to stepS110.

[0117] When the current relation is expressed as Vc=Vr in step S107, thephotosensitive drum 31 is judged to be rotating at the predeterminedrotational speed. In this case, the amount of current passing throughthe driving motor 501 does not need to be changed. Thus, the processingproceeds to step S110, the current state of the switching control by theswitching control unit 582 being maintained.

[0118] The CPU 551 judges in step S110 whether the copy operation hasended. If not (“N” in step S110), the CPU 551 returns to step S103. Theprocesses from steps S103 to S110 are repeated, so that thephotosensitive drum 31 is kept rotating at the predetermined rotationalspeed.

[0119] If judging that the copy operation has ended (“Y” in step S110),the CPU 551 instructs the standard signal generating unit 552 to stopgenerating the standard signal fr and also instructs the switchingcontrol unit 582 to stop the switching control so as to stop therotation of the driving motor 501 (step Sill).

[0120] In the present embodiment, differences in frequency and phasebetween the standard signal and the detection pulse signal are detected.It should be noted here that the rotational speed of the motor can becontrolled in accordance with only a difference in frequency between thestandard signal and the detection pulse signal as is the case with thefirst embodiment.

Third Embodiment

[0121] In the second embodiment, the reduction gear mechanism isemployed for the branching unit that transfers the rotational force ofthe driving motor 501 to both the photosensitive drum 31 and thedeveloping roller 37. In the third embodiment, a magnetic linkingmechanism using inner and outer magnetic rotators is employed for abranching unit. The magnetic linking mechanism is explained below. Notethat a copier of the present embodiment is the same as the copier 1 ofthe second embodiment, except for the construction of the branchingunit. Therefore, the explanation for the same components of the copiersis omitted in the present embodiment.

[0122]FIG. 10 shows a construction of a driving device 520 of thepresent embodiment. The driving device 520 is provided for each pair ofa photosensitive drum and a developing unit, meaning that four drivingdevice 520 are provided in the copier of the present embodiment.Therefore, in FIG. 10, the components are assigned numerals without C,M, Y, or K. The same components as described in the precedingembodiments are assigned the same numerals in the present embodiment.

[0123] As shown in FIG. 10, the driving device 520 is composed of adriving motor unit 500 having a planetary-roller reduction device, afirst drive transfer unit 82, a drive branching unit 84, and a seconddrive transfer unit 85. The first drive transfer unit 82 transfers therotational force of the driving motor unit 500 to the photosensitivedrum 31. The drive branching unit 84 is used for branching therotational force of the driving motor unit 500. The second drivetransfer unit 85 transfers the branched rotational force to thedeveloping roller 37.

[0124] The top end of the output shaft 71 of the driving motor unit 500is connected to the photosensitive drum 31 via a coupling 80, so thatthe rotational force of the driving motor 501 is transferred to thephotosensitive drum 31. The coupling 80, the planetary-roller reductiondevice 502, and the output shaft 71 comprise the first drive transferunit 82.

[0125] The rotational force of the output shaft 71 branches using thedrive branching unit 84 and the branched rotational force is transferredto the developing roller 37 via the second drive transfer unit 85.

[0126]FIG. 11 is a cross-sectional view, taken along the plane of theline C-C of FIG. 10, showing the construction of the drive branchingunit 84 viewed in the direction of the arrows indicated next to the lineC-C. As shown in FIG. 11, the drive branching unit 84 is composed of aninner magnetic ring 841 fixed to the output shaft 71 serving as the mainrotator and an outer magnetic ring 842 fixed inside a driving pulley 851serving as the slave rotator. The inner magnetic ring 841 has aplurality of alternating north and south poles that are set in thedirection of rotation along the outer surface of the output shaft 71.The outer magnetic ring 842 also has a plurality of alternating northand south poles set in the direction of rotation along the inner ifsurface of the driving pulley 851, and is set facing but not in contactwith the outer surface of the inner magnetic ring 841. As can beunderstood, magnetic linking force acts between the inner and outermagnetic rings 841 and 842 due to the magnetic attraction between theopposite poles. As the output shaft 71 rotates, the inner magnetic ring841 fixed to the output shaft 71 also rotates. Together with therotation of the inner magnetic ring 841, the outer magnetic ring 842rotates and so does the driving pulley 851. It is preferable that themagnetic poles of the magnetic ring 841 or 842 are set with the samepitch.

[0127] The drive branching unit 84 has two flanges 76 that are set onthe both sides of the driving pulley 851 and rotate coaxial with theoutput shaft 71 as indicated in FIG. 12. With this construction, thedriving pulley 851 can also rotate coaxial with the output shaft 71.

[0128] In FIG. 10, the second drive transfer unit 85 employs a belttransmission mechanism. This belt transmission mechanism is composed ofthe driving pulley 851 fixed to the outer surface of the outer magneticring 842, a slave pulley 852 fixed to the shaft 47 of the developingroller 37, and a timing belt 853 running over the driving pulley 851 andthe slave pulley 852. The second drive transfer unit 85 is not limitedto the belt transmission mechanism, and may employ a gear transmissionmechanism.

[0129] With the stated construction of the driving system, therotational force of the driving motor 501 is transferred to thephotosensitive drum 31 at a high torque via the planetary-rollerreduction device 502 and the output shaft 71.

[0130] Meanwhile, when the output shaft 71 rotates, the inner magneticring 841 of the drive branching unit 84 accordingly rotates. Togetherwith the rotation of the inner magnetic ring 841, the outer magneticring 842 rotates according to the magnetic linking force and so does thedriving pulley 851. Together with the rotation of the driving pulley851, the slave pulley 852 also rotates via the timing belt 853. As aresult, the rotational force of the driving motor 501 is transferred tothe developing roller 37.

[0131] Accordingly, the photosensitive drum 31 and the developing roller37 do not have to be driven by separate motors. Both the drum 31 and theroller Scan be driven by one motor, the driving motor 501. Consequently,space for setting the rotator driving device can be saved as comparedwith a case where the photosensitive drum 31 and the developing roller37 are driven by separate motors. Now, suppose that load fluctuations,such as vibration, occur to the developing unit 38. In such a case, theload fluctuations at a low frequency are attenuated owing to non-contacteffect or to that the inner and outer rings 841 and 842 rotate togetherstrictly due to the magnetic attraction, not the mechanical tightconnection by means of, such as a gear mechanism. Consequently, the loadfluctuations are prevented beforehand from adversely affecting therotation of the photosensitive drum 31.

[0132] In the present embodiment, the inner and outer magnetic rings 841and 842 are not in contact with each other and magnetically attracted toeach other. However, the magnetic rings 841 and 842 may be in contactwith each other as long as they rotate in relation to each other.Although the magnetic rings 841 and 842 are provided for the drivebranching unit 84 in the present embodiment, they may be provided forthe slave pulley 852 and the shaft 47 of the developing roller 37respectively.

[0133] In the present embodiment, the inner and outer magnetic rings 841and 842 are polarized. However, magnets, such as permanent magnets orelectromagnets, can be set at positions where the inner and outermagnetic rings 841 and 842 face each other.

[0134] In a case where the permanent magnets are used, wiring does notneed to be installed as is the case with the present embodiment and,therefore, the construction can be simplified. Meanwhile, in a casewhere the electromagnets are used, the magnetic attraction can bevariably changed by adjusting the amount of current passing through theelectromagnets.

[0135] In the latter case, when the electromagnets are provided for theouter magnetic ring 842, power can be supplied by means of the followingconstruction. A pair of electrodes used for supplying power to theelectromagnets are each formed in a ring shape and set on the outer sideface of one of the flanges 76 so as to be coaxial with the output shaft71. The pair of electrodes is thus exposed, and brush members used forpower supply are fixed corresponding to the pair of electrodes, with therespective ends of the brush members contacting the electrodes.Accordingly, power can be easily supplied to the outer magnetic ring 842that is being rotating. When the electromagnets are provided for theinner magnetic ring 841, the output shaft 71, for example, may be formedin a hollow-body shaft. Then, a power supplying line connected to theelectromagnets is installed inside the hollow-body shaft. By doing so,the power supply can be easily achieved.

[0136] When the drive branching unit 84 has the construction whereby therotational force of the driving motor 501 is transferred by the magneticattraction, the construction is not limited to the stated examples. Forexample, the inner and outer magnetic rings 841 and 842 may be set inparallel on the output shaft 71 so that the magnetic attraction actsbetween the facing parts of the magnetic rings 841 and 842.

[0137] The setting position of the planetary-roller reduction device 502can be freely determined. To be more specific, the planetary-rollerreduction device 502 may be set on either the upstream side or thedownstream side of the drive branching unit 84 in the direction in whichthe rotational force of the driving motor 501 is transferred.

[0138] The rotational speed of the driving motor 501 is controlled usingthe speed detector 73 of the driving device 520 in the presentembodiment. This control operation is performed in the same way asdescribed in the second embodiment and, therefore, the explanation isomitted in the present embodiment.

[0139] In the second and third embodiments, the photosensitive drums 31Cto 31K and the developing rollers 37C to 37K are described as examplesof the plurality of rotators that are driven by the same rotator drivingdevice. When the copier includes an image holding unit aside from thephotosensitive drum, such as a transfer drum or intermediate transferunit, the rotational force of the driving motor may be transferred tosuch an image holding unit. Similarly, aside from the developing roller37, the branched rotational force may be transferred to a rotator suchas a roller, brush, belt, or the like used for charge, image transfer,cleaning, toner fixing, and paper feeding. It should be obvious that thenumber of rotators to which one rotator driving device transfers therotational force may be equal to or more than three.

[0140] In the preceding embodiments, the description has been given fora case where the present invention is applied to a tandem-type digitalcolor copier. The application of the present invention is not limited tothe described embodiments, and can be applied to an image formingapparatus that has a driving device for rotationally driving a rotator,such as a photosensitive drum. Also, the application of the presentinvention is not limited to a copier employing the electrophotographicmethod, and can be applied to various image forming apparatuses, such asa copier employing the direct-writing method. Additionally, the presentinvention is not limited to be included in the image forming apparatus.The rotator driving device and driving method of the present inventioncan be used for other kinds of appliances that have various rotators.

[0141] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art.

[0142] Therefore, unless such changes and modifications depart from thescope of the present invention, they should be construed as beingincluded therein.

What is claimed is:
 1. A rotator driving device that rotationally drivesa rotator, comprising: a motor; a planetary-roller reduction unit thatoutputs a rotational speed that is reduced with respect to a rotationalspeed of the motor, and transfers a rotational drive force of the motorto the rotator; a speed detector that detects a rotational speed of therotator; and a controller that controls the rotational speed of themotor in accordance with a detection result obtained by the speeddetector.
 2. The rotator driving device of claim 1 further comprising acoupling that couples an output shaft of the planetary-roller reductionunit to a rotational shaft of the rotator.
 3. The rotator driving deviceof claim 2, wherein the coupling is detachable.
 4. The rotator drivingdevice of claim 3, wherein the speed detector is located on an upstreamside of the coupling in a direction in which the rotational drive forceof the motor is transferred.
 5. The rotator driving device of claim 2,wherein the speed detector is set on the rotational shaft of the rotatorbetween the rotator and the coupling.
 6. The rotator driving device ofclaim 2, wherein the rotational shaft of the rotator has a partextending to a downstream side of the rotator in a direction in whichthe rotational drive force of the motor is transferred, and the speeddetector is set on the rotational shaft at the extending part.
 7. Therotator driving device of claim 1, wherein the speed detector is a pulseencoder that outputs a pulse at a frequency corresponding to thedetected rotational speed of the rotator.
 8. The rotator driving deviceof claim 7, wherein the controller controls, in accordance with adifference in frequency between the pulse outputted by the pulse encoderand a standard pulse that corresponds to a predetermined rotationalspeed set for the rotator, the rotational speed of the motor so that therotator rotates at the predetermined rotational speed.
 9. The rotatordriving device of claim 8, wherein the controller has a phase differencedetecting unit for detecting a difference in phase between the pulseoutputted by the pulse encoder and the standard pulse, and controls, inaccordance with each difference in frequency and phase between the pulseoutputted by the pulse encoder and the standard pulse, the rotationalspeed of the motor so that the rotator rotates at the predeterminedrotational speed.
 10. The rotator driving device of claim 1 furthercomprising a drive branching unit that branches the rotational driveforce of the motor and transfers the branched rotational drive force toa different rotator.
 11. The rotator driving device of claim 1 furthercomprising a drive branching unit that branches the rotational driveforce of the motor, the branched rotational drive force being used fordriving a different rotator, wherein the drive branching unit has a mainrotating member and a slave rotating member, the main rotating memberbeing set coaxial with the rotator and so rotating as the rotatorrotates, and the slave rotating member being connected to the differentrotator and rotating together with the main rotating member owing to amagnetic action exerted between the main and slave rotating members. 12.An image forming apparatus that has a plurality of image holding drumson each of which an image is formed, the image forming apparatuscomprising a rotator driving device of claim 1 for each of the imageholding drums, wherein the rotator driving device rotationally drivesthe image holding drum.
 13. A rotator driving device that is provided inan image forming apparatus and that drives first and second rotatorsprovided in the image forming apparatus, the rotator driving devicecomprising: a driving unit that includes a motor and supplies arotational drive force of the motor to the first rotator; a speeddetector that detects a rotational speed of the first rotator; acontroller that controls a rotational speed of the motor in accordancewith a detection result obtained by the speed detector; and a drivebranching unit that branches the rotational drive force of the motor andtransfers the branched rotational drive force to the second rotator. 14.The rotator driving device of claim 13, wherein the driving unitincludes a speed reducer that outputs a rotational speed that is reducedwith respect to a rotational speed of the motor and supplies therotational drive force of the motor to the first rotator, wherein thedrive branching unit is located on an upstream side of the speed reducerin a direction in which the rotational drive force is transferred. 15.The rotator driving device of claim 14, wherein a driving shaft of themotor has a part extending in a direction opposite to the speed reducer,wherein the drive branching unit is connected to the extending part ofthe driving shaft of the motor.
 16. The rotator driving device of claim14, wherein the speed reducer is a planetary-roller reduction unit. 17.The rotator driving device of claim 13, wherein the speed detector is apulse encoder that outputs a pulse at a frequency corresponding to thedetected rotational speed of the first rotator.
 18. The rotator drivingdevice of claim 17, wherein the controller controls, in accordance witha difference infrequency between the pulse outputted by the pulseencoder and a standard pulse that corresponds to a predeterminedrotational speed set for the rotator, the rotational speed of the motorso that the rotator rotates at the predetermined rotational speed. 19.The rotator driving device of claim 18, wherein the controller has aphase difference detecting unit for detecting a difference in phasebetween the pulse outputted by the pulse encoder and the standard pulse,and controls, in accordance with each difference in frequency and phasebetween the pulse outputted by the pulse encoder and the standard pulse,the rotational speed of the motor so that the rotator rotates at thepredetermined rotational speed.
 20. The rotator driving device of claim13, wherein the image forming apparatus has an image holding drum as thefirst rotator, an image being formed on a surface of the image holdingdrum.
 21. A rotator-driving method for driving first and secondrotators, comprising the steps of: rotationally driving the firstrotator using a rotational drive force of a motor, branching therotational drive force using a drive branching unit, and rotationallydriving the second rotator using the branched rotational force;detecting a rotational speed of the first rotator using a speeddetector; and controlling a rotational speed of the motor in accordancewith the rotational speed of the first rotator detected by the speeddetector.
 22. A rotator driving device that drives first and secondrotators, comprising: a motor that supplies a rotational drive force tothe first rotator; and a drive branching unit that branches therotational drive force, the branched rotational drive force being usedfor driving the second rotator, wherein the drive branching unit has amain rotating member and a slave rotating member, the main rotatingmember being set coaxial with the first rotator and so rotating as thefirst rotator rotates and the slave rotating member being connected tothe second rotator and rotating together with the main rotating memberowing to a magnetic action exerted between the main and slave rotatingmembers.
 23. The rotator driving device of claim 22, wherein a part ofthe main rotating member faces a part of the slave rotating member, anda plurality of alternating north and south poles are set on each of theparts in a direction of rotation of the main and slave rotating membersso that the magnetic action is exerted between the facing parts.
 24. Therotator driving device of claim 23, wherein the plurality of alternatingnorth and south poles are set with a same pitch.
 25. The rotator drivingdevice of claim 23, wherein the magnetic action is exerted owing to amagnetic attraction between opposite poles.
 26. The rotator drivingdevice of claim 22, wherein a part of the main rotating member faces apart of the slave rotating member, and electromagnets are set on each ofthe parts so that a plurality of alternating north and south poles areset in a direction of rotation of the main and slave rotating members bya passage of electric current through the electromagnets, the magneticaction being exerted between the facing parts.
 27. The rotator drivingdevice of claim 22 further comprising: a speed reducer that outputs arotational speed that is reduced with respect to a rotational speed ofthe motor, and transfers the rotational drive force of the motor to thefirst rotator, the speed reducer being set between the first rotator andthe motor in a path in which the rotational drive force of the motor istransferred; a speed detector that detects a rotational speed of thefirst rotator; and a controller that controls the rotational speed ofthe motor in accordance with a detection result obtained by the speeddetector.
 28. The rotator driving device of claim 27, wherein the speedreducer is a planetary-roller reduction unit.
 29. The rotator drivingdevice of claim 27, wherein the speed detector is a pulse encoder thatoutputs a pulse at a frequency corresponding to the detected rotationalspeed of the first rotator.
 30. The rotator driving device of claim 29,wherein the controller controls, in accordance with a difference infrequency between the pulse outputted by the pulse encoder and astandard pulse that corresponds to a predetermined rotational speed setfor the rotator, the rotational speed of the motor so that the rotatorrotates at the predetermined rotational speed.
 31. The rotator drivingdevice of claim 30, wherein the controller has a phase differencedetecting unit for detecting a difference in phase between the pulseoutputted by the pulse encoder and the standard pulse, and controls, inaccordance with each difference in frequency and phase between the pulseoutputted by the pulse encoder and the standard pulse, the rotationalspeed of the motor so that the rotator rotates at the predeterminedrotational speed.
 32. An image forming apparatus that has a plurality ofrotators, the plurality of rotators including an image holing drum as afirst rotator and another rotator as a second rotator, the image formingapparatus comprising a rotator driving device of claim 22 for drivingthe first and second rotators.
 33. A rotator-driving method for drivingfirst and second rotators, comprising the steps of: rotationally drivingthe first rotator using a rotational drive force of a motor;transferring the rotational drive force via main and slave rotatingmembers to the second rotator so that the second rotator is rotationallydriven, the main rotating member being set coaxial with the firstrotator and so rotating as the first rotator rotates, and the slaverotating member rotating together with the main rotating member owing toa magnetic action exerted between the main and slave rotating members.